In order to form a thermal spray coating on an inner surface of a cylinder bore of a linerless aluminum cylinder block effective for reducing weight and exhaust treatment of an automobile engine, the inner surface of the cylinder bore needs to be roughened in prespray processing for the purpose of enhancing adhesion of the thermal spray coating.
According to Japanese Patent No. 3780840, an inner surface of a cylinder bore is processed through boring processing and thereby is formed into roughened portions in a screwed-shape while the tips of ridge parts being protruding parts of the roughened portions in the screwed-shape are cut off and formed to have a broken surface of finer roughened portions.